Digital communications comprise, in particular, wire-based communications. The transmission of the communication is performed through a transmission channel sometimes called a propagation channel which links a sender and a receiver. The transmission channel corresponds to the physical link between the sender and the receiver. This link is defined by certain parameters and especially by the nature of the link (wire-based, radio, etc.). Certain intermediate equipment such as couplers or repeaters are considered to form part of the transmission channel. Indeed, this equipment participates solely in the transmission of the physical signal and is transparent from the point of view of the senders and receivers.
The invention considers the context of a system comprising at least one sender device A having local reception capabilities and a remote receiver B. The transmission channel is defined for each sender-remote receiver pair. In a broadcast use, with one sender and several receivers, there are as many transmission channels as receivers.
The transmission may be subjected to various phenomena and especially to reflection phenomena. The signal sent by the sender device is reflected, typically at the level of the terminations and junctions for a wire-based channel or at the level of obstacles for a radio channel. These reflections imply that a part of the power sent by the sender device returns to the level of the sender device. We will use the term echo to designate this returned power at the level of the sender.
The transmission channel between a sender A and a receiver B may be characterized by a set of transfer functions which make it possible to determine the effect of the channel on a signal sent by the sender A: hAA, hBA. In a conventional manner, a transfer function is a series of complex values which express the attenuation and the phase shift of the signal for each frequency. Thus, the transfer function hBA makes it possible to determine the signal received at the point B as a function of the signal sent at the point A while applying an attenuation and a phase shift for the whole set of frequencies. Also, the transfer function hAA makes it possible to determine the signal received at the point A as a function of the signal sent at the point A.
Hereinafter in the document, the transmission function from A to B is called hBA and the echo function from A to A is called hAA. These functions may be variable as a function of frequency.
The invention is applied more particularly to line carrier current transmission, commonly designated by the initials PLT (Power Line Transmission). According to this application, the reflections occur in particular at the electrical interfaces, typically at the level of the wall sockets, branch panels etc. FIG. 1 pertains to a particular electrical installation comprising a sender and a remote receiver. FIG. 1 illustrates the level of the amplitude of the echo function, expressed in dB, at the level of the sender, in the case of a PLT transmission destined for the remote receiver.
The invention is, however, not limited to this type of transmission by carrier current and can find application to other types of wire-based transmission as well as to non-wire-based transmissions, in particular aerial transmissions.
Telecommunication systems are conventionally implemented in a context of national or international regulation. A regulating authority defines a maximum permitted power level Plim,f which is generally translated into the form of a send mask. This mask is fixed and imposes the power of the signal sent Pf. An exemplary fixed send mask is illustrated by FIG. 2. It is associated with the systems in accordance with the HomePlugAV specification defined by a group of members and accessible at the following Internet address http://www.homeplug.org. FIG. 3 illustrates the send mask associated with the systems in accordance with the SDSL standard of the ITU.
With reference to FIGS. 4a, 4b, 4c, a known full-duplex multicarrier digital communication system of SDSL type comprises a sender/receiver device ERA and a remote sender/receiver device ERB, that are separated by a transmission channel CT.
The multicarrier signal sent has a power spectral density distributed over the send band split up into carriers according to the characteristics of the telecommunication system.
FIGS. 4a, 4b, 4c represent in a schematic manner certain modules of the sender/receiver device ERA and of the sender/receiver device ERB, for various instants of a process of sending a multicarrier signal between the sender/receiver device ERA and the sender/receiver device ERB. The sender/receiver device ERA comprises a send module EMEA, a receive module RECA, a module ESTBA for estimating the transmission function hBA, a module ESTAA for estimating the echo function h′AA, an echo cancellation module ANA, a channel equalization module EGAA. The sender/receiver device ERB comprises a send module EMEB, a receive module RECB.
Upon the sending of a first particular multicarrier sequence seq1—A by the send module EMEA, FIG. 4a, at a power level determined according to a fixed power mask mqf1—A, the transmission channel CT transmits the signal sent, a part of which is received by the receive module RECB. A part of the power of the signal sent is returned and received by the receive module RECA. This part corresponds to the signal sent seq1—A attenuated and phase-shifted for the set of frequencies by the echo function h′AA. The estimation module ESTAA estimates this echo function knowing the signal sent seq1—A and the returned signal received seq1—A_h′AA provided by the receive module RECA.
Upon the sending of a second particular multicarrier sequence Seq2—B, FIG. 4b, known to the sender/receiver device ERA, at a power level determined according to a fixed power mask mqf1—B, by the send module EMEB of the sender/receiver device ERB, the signal transmitted by the transmission channel CT is received by the receive module RECA of the sender/receiver device ERA. This received signal corresponds to the second sequence Seq2—B attenuated and phase-shifted for the set of frequencies by the transmission function h′AB. The estimation module ESTAB estimates this transmission function h′AB knowing the second particular multicarrier sequence seq2—B and the transmitted signal received provided seq2—B_h′AB by the receive module RECA.
During a full-duplex transmission, FIG. 4c, each of the two send modules EMEA and EMEB of the sender/receiver devices respectively ERA and ERB simultaneously send a multicarrier signal SeA, SeB, at a power level determined according to a respective fixed power mask mqf2—A, mqf2—B. The receive module RECA of the sender/receiver device ERA receives the signal transmitted by the transmission channel CT on account of the send by the sender/receiver device ERB and the echo on account of the simultaneous send by the sender/receiver device ERA. The echo cancellation module ANA cancels this echo of the signal received. The cancellation consists in subtracting the echo received from the signal received SeA×hAA+SeB×hAB provided by the receive module RECA. To evaluate the echo received SeA—hAA=SeA×hAA, the cancellation module ANA makes the approximation that the echo function hAA is identical to the echo function h′AA estimated during the sending of a first known sequence. The cancellation module ANA therefore subtracts the signal sent SeA attenuated and phase-shifted for the set of frequencies by h′AA, from the signal received. The channel equalization module EGAA evaluates the signal sent SeB, for example by dividing the signal provided at the output of the cancellation module ANA by the coefficient h′AB. The equalization module EGAA makes the approximation that the transmission function hAB is identical to the transmission function h′AB estimated during the sending of a second particular sequence seq2—B.
In this known multicarrier digital communication system of SDSL type, the echo cancellation module intervenes during communications in full-duplex mode.
The PEB 22622 (SOCRATES) component from the company Infineon is adapted for an SDSL system and comprises the modules of a previously described sender/receiver device.
The echo, resulting from the reflection phenomenon occurring during transmission, may disturb the receive module of a sender/receiver device, in a full-duplex mode. To combat this disturbance, the receive module of the sender/receiver device cancels the echo, by means of an echo cancellation module, by estimating the echo by taking account of a previously estimated echo function and of the signal sent.
As regards the signal received by the receive module of a remote sender/receiver device, it is diminished with respect to the signal sent by the power of the echo.